The present invention relates to an optical head for controlling an output from a light source using a front monitor system, and an optical reproduction apparatus and an optical record and reproduction apparatus which uses the optical head installed therein.
Outputs from a light source of an optical head used in an optical record and reproduction apparatus need to be controlled accurately and stably with respect to change in environment such as temperature and elapse of time in recording and reproducing of various types of optical information recording media. Since outputs from a semiconductor laser generally used as a light source in optical heads fluctuate due to a change in temperature or aging, in order to have a stable output from the light source constantly, it is a general practice to attempt to stabilize the power level of a beam of light emitted to an optical information recording medium such as an optical disk by implementing power control through APC (Auto Power Control).
As systems representative of the APC, there are known a rear monitor system and a front monitor system. The rear monitor system is a system in which beams of light emitted from a rear side of a semiconductor laser chip towards the interior of a laser package are detected by a photodetector provided in the interior of the package. In this system, since the photodetector is provided inside the laser package, the optical head can be made smaller in size. Currently, this rear monitor system is used in reproduction only apparatuses or those in exclusive use for reproduction.
In the rear monitor system, however, since beams of light emitted from an end face of the semiconductor laser which is opposite to a radiating surface thereof, there has been a problem that the detection accuracy deteriorates. Consequently, the front monitor system is used in recording types which require high accuracy. The front monitor system is a system in which part of beams of light emitted from the semiconductor laser, which is the light source, is split, so that split light is detected to be fed back to a driving circuit of the semiconductor laser, whereby the output of the semiconductor laser is controlled by virtue of the intensity of the split light. Specifically, part of light emitted from the semiconductor laser is received by a front monitoring photodetector for feedback control, and the power control is implemented through APC. Note that since the front monitor system is more accurate than the rear monitor system, the front monitor system may also be used for reproduction only apparatuses.
Incidentally, in order to implement APC with good accuracy, it is desirable that a quantity of light that is incident on the front monitoring photodetector and a quantity of light that is actually used in record and reproduction of information are at a constant ratio at all times. Normally, laser beam that is used in recording and reproducing is linear polarized light traveling in a direction of a TE wave which is parallel with an interface of an active layer of the semiconductor laser (hereinafter, referred to as TE polarized light).
On the other hand, a spontaneous emission component in which phase and polarization are not in regulated is also emitted from the semiconductor laser. Since the spontaneous emission component is emitted widely from the semiconductor laser, when compared with an emitted light made up of a laser beam with less divergence, the far field pattern (hereinafter, referred to as FFP) expands widely. The spontaneous emission component is light which is not used in record and reproduction of information, and in the event that the spontaneous light reaches to an optical information recording medium to be reflected thereon or is reflected irregularly in the interior of a casing of the optical head or other optical components, the light so reflected constitutes a kind of so-called stray light. In the event that stray light reaches a detector for reproduction signals or servo signals for the optical information recording medium, it constitutes a cause for deterioration in quality of various types of signals, representative of increase in noise and deterioration in jitter.
Here, the output properties of semiconductor lasers that are used in record and reproduction of information will be described while referring to FIG. 7. FIG. 7A shows an example of the output properties of a semiconductor laser of GaAs system (a red semiconductor laser), and FIG. 7B shows an example of the output properties of a semiconductor laser of nitride system (violet semiconductor laser), represented by GaN, which has been started to be used in recent years.
In FIGS. 7A, 7B, a dotted line C is an extension from the slope of a straight line before the radiation of a laser beam and represents the output of a spontaneous emission component contained in an emitted light. In addition, a dotted line A and a dotted line B are such as to represent values of driving currents which are needed in reproduction and record of information, respectively. A spontaneous emission component is emitted before the radiation of a laser beam, and in a polarization state as this occurs, the intensity of both TE polarization component and TM polarization (a polarization which vibrates in a direction perpendicular to TE polarization) component is substantially identical. On the other hand, after a laser beam is radiated by being driven by a sufficiently large current, most component is TE polarization.
Generally, since a large output of laser beam is necessary in recording information, the driving current becoming large, there will be no serious problem even in the event that TE polarization constitutes most of the radiation of a laser beam (the dotted line B). However, an output necessary in reproducing the information is smaller than the output needed in recording and is on the order of a magnitude which is slightly larger than the output of a current necessary to start the radiation of a laser beam (the dotted line A). In a region like this, a ratio of the spontaneous emission component relative to the total output is relatively large, and hence the spontaneous emission component needs to be taken into consideration. In addition, as shown in FIG. 7, since the spontaneous emission component is larger in the semiconductor laser of GaN system than in the semiconductor laser of GaAs system, in particular, the effect of the spontaneous emission component becomes large.
Here, referring to FIGS. 8 and 9, a conventional optical head adopting the front monitor system (for example, Japanese Patent No. 2555239) will be described. FIG. 8 is a side view illustrating the schematic configuration of the conventional optical head and FIG. 9 is a plan view illustrating the schematic configuration of the conventional optical head.
As shown in FIGS. 8 and 9, a beam of light emitted from a light source 1 made up of a semiconductor laser is split into three beams of light for generating a tracking error signal at a diffraction device 2, and the three beams of light so split are then made to be parallel beams of light by a collimator lens 3 so as to be incident on a beam splitter 4a as beams of P polarized light. Note that, relative to the beam splitter 4a shown in FIG. 9, polarized light which vibrates in a parallel direction to the surface of a sheet of paper on which FIG. 9 is drawn is assumed as P polarized light, and polarized light which vibrates in a perpendicular direction relative to the surface of the sheet of paper is assumed as S polarized light. In addition, the light source 1 is disposed such that the direction of TE polarized light emitted from the light source 1 and the direction of P polarized light coincide with each other. The beam splitter 4a has, for example, properties in which on the order of 90% of P polarized light is transmitted therethrough and the remaining 10% thereof is reflected thereon.
As shown in FIG. 8, beams of light which have been transmitted through the beam splitter 4a are reflected by a reflecting mirror 5 for rise, whereby the optical path thereof is directed. Then, the beams of light are then made into beams of circular polarized light when passing through a ¼ wave-length plate 6 and is then incident on an objective lens 7. The beams of light are then made to be convergent beams at the objective lens 7, so that the beams of light so converged are then converged on an information track on an information recording surface of an optical information recording medium 8. Note that the objective lens 7 is installed on an actuator 9 which is movable in at least focusing and tracking directions relative to the optical information recording medium 8.
Beams of light reflected on the information recording surface of the optical information recording medium 8 transmit through the objective lens 7 and are then converted into beams of S polarized light (beams of linear polarized light which intersect at right angles with an outbound path from the light source 1 to the ¼ wave-length plate 6) at the ¼ wave-length plate 6. Then, the beams of light so converted are reflected by the reflecting mirror 5 so as to be incident on the beam splitter 4a as the beams of S polarized light.
Since the beam splitter 4a reflects almost 100% of the S polarized light, beams of light that are reflected by the beam splitter 4a, as shown in FIG. 9, are made to be convergent beams of light at an imaging lens 10 and are given an astigmatic aberration for generating a focus error signal at an anamorphic lens 11. Then, the beams of light so given the astigmatic aberration are incident on a photodetector 12 so as to be converted into an electric signal at a light receiving portion thereof
On the other hand, beams of light, which constitute part of the beams of light emitted from the light source 1 and are reflected by the beam splitter 4a, enter a front monitoring photodetector 13, as shown in FIG. 9. Then, the beams of light are converted into an electric signal for monitoring outputs from the light source at a light receiving portion of the photodetector 13, and the output monitoring electric signal so converted is then used for feedback control of outputs from the light source 1.
On the other hand, various methods have been proposed in which APC is implemented using part of beams of light which are out of the effective range and hence are not used in information record and reproduction (for example, Patent Document JP-A-2001-118281 and JP-A-10-255314). Here, the “beams of light which are out of the effective range” means beams of light which are other than beams of light which fall within the effective range and hence are effectively used in information record and reproduction.
Referring to FIG. 10, an optical head according to the related art will be described. FIG. 10 is a plan view illustrating the schematic configuration of a conventional optical head. A photodetector 13 is disposed on an optical path for peripheral beams of light.
As shown in FIG. 10, beams of light emitted from a light source 1 which have a highest intensity and which are located substantially centrally (beams of light which fall within the effective range and which are hence effectively used in information record and reproduction) are incident on a diffracting device 2, where the beams of light are split into three beams of light for generating a tracking error signal. Then, the beams of light so split are made to be parallel beams when they transmit through a collimator lens 3 and are then incident on a beam splitter 4b. In addition, the light source 1 is disposed such that the direction of TE polarized light emitted from the light source 1 and the direction of P polarized light in the beam splitter 4b coincide with each other.
The beam splitter 4b has properties to transmit nearly 100% of the P polarized light and to reflect nearly 100% of the S polarized light. Consequently, beams of P polarized light are allowed to transmit through the beam splitter 4b, whereas beams of S polarized light are reflected by the beam splitter 4b. 
Similarly to the related art that have already been described above, beams of P polarized light, which have transmitted through the beam splitter 4b, are reflected by a reflecting mirror 5 and are made into beams of circular polarized light when they transmit through a ¼ wave-length plate 6. Thereafter, the beams of light enter an objective lens 7 and are then converged on an information track on an information recording surface of an optical information recording medium 8. The beams of light, which are reflected on the information recording surface of the optical information recording medium 8, are converted into beams of S polarized light (beams of linear polarized light which intersect at right angles with an outbound path from the light source 1 to the ¼ wave-length plate 6) at the ¼ wave-length plate 6, so that the beams of light so polarized are incident on the beam splitter 4b as S polarized light.
Since the beam splitter 4b has the properties to reflect nearly 100% of the S polarized light, the beams of S polarized light which have been reflected on the beam splitter 4b transmit along an imaging lens 10 and an anamorphic lens 11 so as to be incident on a photodetector 12. The photodetector 12 converts the beams of S polarized light into an electric signal in accordance with a quantity of light the photodetector 12 has received. A predetermined operation is applied to an electric signal resulting from the conversion so as to generate a focus error signal, a tracking error signal and a reproduction signal.
On the other hand, beams of light that are emitted from the light source 1 and which constitute part of beams of light which are out of the effective range are not incident on a diffracting device 2 but are incident on a photodetector 13 which is similarly disposed out of the effective range. Then, the beams of light are converted into an electric signal for monitoring outputs from the light source 1 by a light receiving portion of the photodetector 13, and an output monitoring electric signal resulting from the conversion is used for feedback control of outputs of the light source 1.
In the case of the optical head shown in FIGS. 8 and 9, what is converged on to the optical information recording medium 8 for recording and reproducing of information is made up only of the laser beam, which is P polarized light (=TE polarized light), while, of the light emitted from the light source 1, 10% of the P polarized light and 100% of the S polarized light are incident on the photodetector 13. Since the ratio of P polarized light and S polarized light changes depending on the driving current of the light source 1, the intensity ratio between the beams of light that has transmitted through the beam splitter 4b for information record and reproduction and the beams of light for APC (beams of light which are received by the photodetector 13) in which the ratio of S polarized light, which is the spontaneous emission component, is increased due to the reflection on the beam splitter 4b does not become constant relative to optical outputs from the light source 1. Due to this, there is generated an error in APC.
Here, the error so generated will be described specifically. In a case where intensities with which beams of light, which are converged on to the information recording surface of the optical recording medium 8 to form beam spots directly used for recording and reproducing of information, are emitted onto the optical information recording medium 8 are 4.5[mW] and 0.45[mW], respectively, assuming that the light utilization efficiency of the optical system for the optical head excluding the beam splitter 4b is 15%, since the transmittance of the beam splitter 4b for the P polarized light (=TE polarized light) is 90%, the output of the laser beam source necessary at the time of record becomes on the order of 33[mW], and that necessary at the time of reproduction becomes 3.3[mW]. This state substantially correspond to the dotted lines B, A shown in FIG. 7B, respectively. In addition, the spontaneous emission component emitted from the light source 1 corresponds to the dotted line C in FIG. 7B, and since TE polarized light and TM polarized light are contained therein in substantially the same quantity, the intensity of both TE polarized light and TM polarized light in the spontaneous emission component outputted at the time of record becomes on the order of 0.92[mW], and the intensity of both TE polarized light and TM polarized light in the spontaneous emission component outputted at the time of reproduction become on the order of 0.77[mW].
On the other hand, as to the front monitoring photodetector 13, in consideration of FFP, the vignetting factor of the light receiving portion of the photodetector 13 is made to be 4.0% for laser beam and 3.0% for spontaneous emission component. Since the reflectance of the beam splitter 4b is 10% for P polarized light (=TE polarized light) and 100% for S polarized light (=TM polarized light), a luminous intensity detected at the photodetector 13 at the time of record is;
            (              laser        ⁢                                  ⁢                  beam          ⁡                      (                          TE              ⁢                                                          ⁢              polarized              ⁢                                                          ⁢              light                        )                              )        +          (              spontaneous        ⁢                                  ⁢        emission        ⁢                                  ⁢        component        ⁢                                  ⁢                  (                      TE            ⁢                                                  ⁢            polarized            ⁢                                                  ⁢            light                    )                    )        +          (              spontaneous        ⁢                                  ⁢        emission        ⁢                                  ⁢        component        ⁢                                  ⁢                  (                      TM            ⁢                                                  ⁢            polarized            ⁢                                                  ⁢            light                    )                    )        =                    (                  33          ×          4.0          ⁢          %          ×          10          ⁢          %                )            +              (                  0.92          ×          3.0          ⁢          %          ×          10          ⁢          %                )            +              (                  0.92          ×          3.0          ⁢          %          ×          100          ⁢          %                )              =          0.16      ⁢                          [      mW      ]      
In addition, similarly, a luminous intensity detected at the photodetector 13 at the time of reproduction is;
                    =                ⁢                              (                          3.3              ×              4.0              ⁢              %              ×              10              ⁢              %                        )                    +                      (                          0.77              ×              3.0              ⁢              %              ×              10              ⁢              %                        )                    +                                                ⁢                  (                      0.77            ×            3.0            ⁢            %            ×            100            ⁢            %                    )                                        =                ⁢                  0.04          ⁢                                          [          mW          ]                    
Due to this, the ratio of the quantity of light that reaches the optical information recording medium 8 to be actually used for record and reproduction of information to the quantity of light detected by the photodetector 13 becomes;
0.16/4.5=3.6% at the time of record, and
0.04/0.45=8.6% at the time of reproduction, whereby the ratios become different largely between the time of record and the time of reproduction.
Thus, in the case of the semiconductor laser of GaN in which the spontaneous emission constant is large, since the ratio between the quantity of light for record and reproduction and the quantity of light for APC changes depending on the magnitude of the quantity of emitted light (driving current), there is generated an error in APC. In addition, as with the optical system shown in FIGS. 8 and 9, in the optical system in which parallel beams are incident on the beam splitter 4a by means of the collimator lens 3, the angle of incidence of a ray of light relative to the splitting surface of the beam splitter 4b becomes equal over the whole area of the section of the beam of light. Consequently, since the reflectance of P polarized light and S polarized light becomes equal over the whole area of the section of the beam of light, even in the event that the area and shape of the light receiving portion of the front monitoring photodetector 13 are changed, or the photodetector 13 itself is caused to deviate to be disposed, it is difficult to reduce the error.
In addition, in the case of the optical head shown in FIG. 10, while the photodetector 13 receives part of the beams of light which are out of the effective range and are hence not used for recording and reproducing of information, since, in the beams of light which are out of the effective range, the intensity ratio of the spontaneous emission component in which FFP expands largely is increased, the error in APC is increased.
Here, the error so generated will be described specifically. In a case where intensities with which beams of light, which are converged on to the information recording surface of the optical recording medium 8 to form beam spots directly used for recording and reproducing of information, are emitted onto the optical information recording medium 8 are 4.5[mW] and 0.45[mW], respectively, assuming that the light utilization efficiency of the optical system for the optical head excluding the beam splitter 4b is 15%, since the transmittance of the beam splitter 4b for the P polarized light (=TE polarized light) is 100%, the output of the laser beam source necessary at the time of record becomes on the order of 30[mW], and that necessary at the time of reproduction becomes 3[mW]. This state substantially correspond to the dotted lines B, A shown in FIG. 7B, respectively. In addition, the spontaneous emission component emitted from the light source 1 corresponds to the dotted line C in FIG. 7B, and since TE polarized light and TM polarized light are contained therein in substantially the same quantity, the intensity of both TE polarized light and TM polarized light in the spontaneous emission component outputted at the time of record becomes on the order of 0.90[mW], and the intensity of both TE polarized light and TM polarized light in the spontaneous emission component outputted at the time of reproduction become on the order of 0.77[mW].
On the other hand, as to the front monitoring photodetector 13, in consideration of FFP out of the effective range, the vignetting factor of the light receiving portion of the photodetector 13 is made to be 0.5% for laser beam and 1.0% for spontaneous emission component. When setting like this, a luminous intensity detected at the photodetector 13 at the time of record is;
            (              laser        ⁢                                  ⁢                  beam          ⁡                      (                          TE              ⁢                                                          ⁢              polarized              ⁢                                                          ⁢              light                        )                              )        +          (              spontaneous        ⁢                                  ⁢        emission        ⁢                                  ⁢        component        ⁢                                  ⁢                  (                      TE            ⁢                                                  ⁢            polarized            ⁢                                                  ⁢            light                    )                    )        +          (              spontaneous        ⁢                                  ⁢        emission        ⁢                                  ⁢        component        ⁢                                  ⁢                  (                      TM            ⁢                                                  ⁢            polarized            ⁢                                                  ⁢            light                    )                    )        =                    (                  30.0          ×          0.5          ⁢          %                )            +              (                  0.90          ×          1.0          ⁢          %                )            +              (                  0.90          ×          1.0          ⁢          %                )              =          0.17      ⁢                          [      mW      ]      
In addition, similarly, a luminous intensity detected at the photodetector 13 at the time of reproduction is;
                    =                ⁢                              (                          3              ×              0.5              ⁢              %                        )                    +                      (                          0.77              ×              1.0              ⁢              %                        )                    +                      (                          0.77              ×              1.0              ⁢              %                        )                                                  =                ⁢                  0.03          ⁢                                          [          mW          ]                    
Due to this, the ratio of the quantity of light that reaches the optical information recording medium 8 to be actually used for record and reproduction of information to the quantity of light detected by the photodetector 13 becomes;
0.17/4.5=3.7% at the time of record, and
0.03/0.45=6.7% at the time of reproduction, whereby the ratios become different largely between the time of record and the time of reproduction.
Thus, in the case of the semiconductor laser of GaN in which the spontaneous emission constant is large, since the ratio between the quantity of light for record and reproduction and the quantity of light for APC changes depending on the magnitude of the quantity of emitted light (driving current), there is generated an error in APC.
In the event that there is generated an error in APC and the laser beam output (power) scatters, a problem tends to be caused, in particular, in a writable optical information recording medium such as a phase changeable disk. In the case of the phase changeable disk, since the storing power is changed over at high speeds and the deleting power is set, with a poor accuracy with which a laser beam is outputted, the generation of recording error is easy to occur.
In the reproduction of a writable optical information recording medium, in the event that the reproducing power deviates toward a higher side, there may be caused a risk of occurrence of erroneous deletion. In addition, when reproducing a ROM medium having high reflectance, in the event that the reproducing power scatters largely, the magnitude of an electric signal, which is subjected to photoelectric conversion and amplified in a photodetector for reproduction signal and servo signal for an optical information recording mediun, scatters largely, and due to the signal waveform being distorted or being too small, the generation of a read error is facilitated.
Furthermore, since the effect of the aforesaid stray light is increased not only due to the problem with the APC error but also due to the spontaneous emission component of the semiconductor laser of GaN system being large, stray light reaches the photodetector for reproduction signal and servo signal for the optical information recording medium, whereby the deterioration in quality of various signals such as an increase in noise or deterioration in jitter tends to be called for easily.